fix: clear terminal loop actions

[cheshirecode]

Summary

• clear inherited next_action when a loop finishes complete or cancelled
• require explicit next actions for resumable terminal outcomes and reject reopening guidance on non-resumable annotations
• document the terminal contract and add atomic regression coverage

Verification

• bash tests/run.sh all — 65 pass, 0 fail
• loop-state fixtures — 17 pass
• disposable CLI dogfood verifies empty completed action and atomic blocked rejection
• ruff format/check, Skill Creator validation, and git diff --check